Method and apparatus for selective treatment of a precision substrate surface

ABSTRACT

The present invention may be used in the field of precision manufactured and assembled devices and precision test apparatus, among many others, for the selective preparation (cleaning and modification) of a critical substrate surface prior to, during, or following manufacturing and assembly operations such as coating, bonding, patterning, sealing, dicing, cutting, drilling, polishing, planarization, among many other manufacturing operations. Moreover, the present method may be used to prepare medical instruments for implant or use and for dry cleaning textile substrates by providing a combination of surface cleaning and modification using low pressure plasma in combination with plasma-dense fluid surface cleaning pre-treatments. The present invention illustrates a method with various exemplary apparatuses for developing and employing an instant enabling, dry, and selective surface cleaning and modification method using two or more advanced dry cleaning techniques; plasma cleaning, dense fluid cleaning, electrohydrodynamic cleaning and UV/Ozone cleaning. Using two or more of these techniques sequentially or simultaneously as an instant surface treatment combination, virtually any type of contamination may be efficiently, effectively, and selectively removed from a precision substrate surface without damaging said beneficial surface features. Following which the precision substrate surface may be modified, coated, assembled, or used.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This invention claims the benefit, under Title 35, United StatesCode 119 (e), of Provisional Application No. 60/377,197, filed May 3,2002 entitled “Method and Apparatus for Selective Treatment of aPrecision Substrate Surface” which is hereby incorporated by thisreference.

BACKGROUND OF INVENTION

[0002] As technology has advanced, the system performance requirementsand complexity of manufactured and assembled precision instruments anddevices have increased, while the size of individual components andassemblies have decreased. This continuing miniaturization process hasmagnified the susceptibility of precision substrates and surfaces tocontamination. Specific effects of contamination depend on the type ofsubstrate, materials used, and system in which the device is used.

[0003] Thus, an important element of any precision manufacturing orassembly process is the removal of contaminants (oils, particulates,moisture, etc.) from the surfaces of precision substrates. Precisionsubstrate surfaces include those in the manufacturing and assembly ofsemiconductors, fiber optic, optoelectronic, medical, and sensordevices, fabrics, textiles, and instruments, among many othercontaminant-yield sensitive devices. The contamination of these surfacesusually results from external sources such as process equipment,personnel, process reaction by-products, chemical impurities andassembly residues.

[0004] For example, contaminants in contact with a precision substratesurface may inhibit the movement of a microelectromechanical system(MEMS) component such as a gear or moveable mirror, interfere with thetransmission of light, prevent uniform electrodeposition of a metal,prevent wetting of a bonding agent, decrease adhesion strength betweenbonding interfaces, or produce shorts in microscopic electronicinterconnects.

[0005] More than 80% of the yield loss of volume-manufactured integratedcircuits is attributable to particle contamination. As device geometriescontinue to shrink and wafer size increases, particulate matter andresidues will have an ever-increasing impact on device yields. Currentcleaning technologies become less effective with the growing demand forremoving sub-micron (<1 micrometer) contamination.

[0006] Selection of an appropriate combination of cleaning techniquesfor precision substrate surface cleaning must include consideration ofthe type of soil to be removed, substrate composition and properties,and the desired level of cleanliness. Precision substrate surfacecleaning involves a wide range of substrate materials of composition,including metals, fibers, colorants, pigments, polymers, plastics,epoxies, and sealants, and usually have stringent cleanlinessrequirements such low particle counts, no surface residues, wetability,and surface brightness. Furthermore, a wide range of contaminationsexist, including particle contamination, chemical contamination,biological contamination, ionic contamination, molecular contaminationand outgassing or offgassing contamination. In addition, the precisionsubstrate surfaces can exhibit a variety of surface geometries (e.g.,tubing, insulated wires, small orifices, surface topography) that canmake efficient cleaning and drying very difficult to achieve. To. date,no known universal and effective dry cleaning alternative method for thevariety of precision substrate surface preparations exists. To properlydevelop such a universal alternative cleaning methodology, eachalternative must be evaluated based on all of the above cleaning factorsas well as cleanliness requirements, material compatibility, and costeffectiveness specific to the desired surface cleaning application.

[0007] For example, aqueous cleaning may be effectively used to cleansimple geometric surfaces, but the cleaning liquid may become entrappedin crevices and hidden cavities of more complex components, thuspotentially affecting the operation of the precision device if notthoroughly removed in subsequent drying steps. Furthermore, aqueouscleaning techniques are not compatible with new substrate materials andshrinking geometric features, which leads to effects such as corrosionor oxidation or may cause substrate damage during drying operations dueto capillary force pressure within microvias and cavities.

[0008] It important to discuss the specific types of contaminations andprecision substrate surfaces encountered and cleaning energies requiredto perform the myriad number of precision cleaning operations, andaddressed by the present invention. The following is a generalizedcategorization of common surface contaminations and substratesencountered in precision cleaning applications. Surface contaminationsmay be classified and typified into four major groups according toTable 1. TABLE 1 Contamination Classification Scheme Class A—Thick Films(viscous liquids and solids having micrometer level thickness) TypeA1—Cross linked and bonded films (i.e., optical coatings, polymers,plasma etch residues) Type A2—Viscous organic films (i.e., hydrocarbons,stains, blood) Class B—Thin Films (viscous liquids having monolayer andnanometer level thickness) Type B1—Organic films (i.e., resist residue,finger oils, trace hydrocarbons, biological, haze) Type B2—Inorganicfilms (i.e., water, minerals, ionics, oxides) Class C—Particulates(inorganic and organic solids and semi-solids) Type C1—Macroscopic(i.e., particle sizes > 10 micron) Type C2—Microscopic (i.e., particlessizes between 0.5 and 10 microns) Type C3—Nanoscopic (i.e., particlesizes < 0.5 microns) Class D—Outgassing Compounds (condensed vapors)Type D1—Organic outgassing compounds (i.e., organic gases and vapors)Type D2—Inorganic outgassing compounds (i.e., inorganic gases and watervapor)

[0009] Precision Substrate Surfaces may be classified and typifiedaccording to Table 2. TABLE 2 Substrate Surface Classification SchemeClass A—Anisotropic Surface Type A1—Planer (i.e., diced or whole diskdrive, semiconductor, optical and MEMS wafers) Type A2—3-Dimensional(i.e., optical benches, CMOS image sensor, implantable device) ClassB—Isotropic Surface Type B1—Planer (i.e., photodiode, LCD, lead frame,optical lenses, polyimide film, IC test socket pad) TypeB2—3-Dimensional (i.e., optical fiber, hollow tube, CMOS image sensor)

[0010] Cleaning is defined as the removal of unwanted substances(Table 1) from a substrate surface or subsurface (Table 2). The processof removing unwanted substances involves breaking bonds, chemical andphysical, using a combination of mechanical, physical, and chemicalenergy. Furthermore, cleaning performance is measured in terms of acombination of the aforementioned cleaning energies, level ofcontamination, and cleaning time required to meet a certain cleanlinesslevel. The mechanics of cleaning are complex due to the many variablesas discussed above, thus an innumerable variety of conventional wet anddry combinational cleaning methodologies exist.

[0011] Furthermore, cleaning energies may be classified and typifiedaccording to Table 3. TABLE 3 Cleaning Energies Class A—MechanicalEnergy Type A1—Shear stress Type A2—Acoustic Class B—Physical EnergyType B1—Heat Type B2—Surface tension Type B3—Viscosity Class C—ChemicalEnergy Type C1—Solubilization Type C2—Oxidation

[0012] The proper selection and application of these cleaning energies(Table 3) is critical to efficiently remove of the many varieties ofcontaminants encountered in precision cleaning applications, reducingcleaning time, meeting cleanliness requirements, and selectivelytreating without damage to a precision substrate surface.

[0013] As can be seen from Tables 1, 2 and 3, removal of the varioustypes of contaminants from precision substrate surfaces necessitates theuse of several cleaning energies. For example, particles (i.e., Table 1,Type C1, C2, and C3 contaminants) contained on a wafer surface (i.e.,Table 2, Type A1 substrate surface) require shearing action (i.e., Table3, Type A1 cleaning energy) for complete removal. However, plasmareacted resist residues (i.e., Table 1, Type B1 contaminants) on thatsame wafer would require additional cleaning energy in the form ofoxidative cleaning species (i.e., Table 3, Type C2 cleaning energy) toachieve both a particle and residue free substrate surface. Also, othercleaning energies (i.e., solubility, thermal, and shear) may be requiredto rinse and dry the cleaned substrate using, for example, aconventional wet and dry cleaning method.

[0014] In conventional processes, several wet and dry cleaningtechniques are employed to achieve the desired quality and performanceof the cleaning process. This is so because each cleaning techniquedelivers a certain and usually fixed performance profile—that is a typeof cleaning energy and effectiveness for a certain type and level ofcontamination and for a certain type of substrate surface. Also becausewet and dry methods are different chemically and physically, specialrinsing and drying techniques must be included in the methodology.

[0015] For example, a common technique used to achieve various levels ofcleanliness, be it organic, inorganic and particulate cleanliness,involves the combinational use of various wet and dry cleaning anddrying technologies. Examples of combinational cleaning and dryingprocesses include organic solvent cleaning (i.e., Table 3, Type A1 andC1 cleaning energies) followed by nitrogen drying (i.e., Table 3, TypeA1 and B1 cleaning energies); oxidative hydroxylamine cleaning (i.e.,Table 3, Type C2 cleaning energy) followed by deionized water flushing(i.e., Table 3, Type A1 and C1 cleaning energies) and alcohol drying(i.e., Table 3, Type C1 and B3 cleaning energies); and plasma cleaning(i.e., Table 3, Type C2 cleaning energy) followed by ozonated waterresidue removal and alcohol drying (i.e., Table 3, Type A2, B3, and C2cleaning energies).

[0016] As can be seen, conventional cleaning and drying methods can befairly extensive and most often involve combinations of wet and drychemistries, techniques and equipment. However, conventional cleaningmethods are becoming increasingly problematic for precision substratesurfaces as device geometries shrink and new manufacturing materials areused. For example, issues such as microscopic and contaminant-relateddefects caused by stiction and capillary collapse are becoming moreprevalent in optical and IC wafer fabrication. Furthermore, highlyenergetic cleaning processes such as vacuum plasmas may damagesubstrates and especially microscopic features present on a surfacewhile removing unwanted surface contaminations. Still moreover, toachieve nano-scale levels of cleanliness without damage to the substratesurface and to modify said substrate surface to prepare for followingoperations, a many iterations of wet and dry processing is required,each process enabling the next produce a biocompatible surface, producea hydrophobic surface, or to create a barrier film. Surfacemodifications can only be properly performed on a precision substrateonce its surface is free of hydrocarbons, particles and othercontaminating residues. As such, there is a present need for a drycombinational cleaning and surface treatment method that can produce aclean surface first and then modify said cleaned surface to produceadditional beneficial surface properties as identified in Table 4.

[0017] To address this need, the present inventor has developed acompletely dry substrate surface cleaning and modification method usinga unique combination of state-of-the-art dry cleaning and surfacemodification technologies. Candidate technologies were identified,studied, and evaluated to determine the performance characteristics andlimitations for each.

[0018] As a result of this work, it has been discovered that usingvarious dry cleaning and surface modification techniques in certaincombinations, called instant surface treatment methods herein, allowsfor complete treatment of a substrate surface without resorting toconventional wet cleaning and drying methods described above. Thepresent method provides the entire range of cleaning energies (i.e.,Table 3) required for the various contaminations (i.e., Table 1) andsubstrate surfaces (i.e., Table 2) encountered in most precisionsubstrate surface cleaning. Moreover, a cleaning technique was chosen(i.e. low pressure plasma) which serves as a follow-on surfacemodification technique (i.e., Table 4), thus increasing the utility ofthe present invention. The present method is highly selective and aninstant method may be constructed so as not to damage delicate featuresfound on precision substrate surfaces. In many applications, only theaffected substrate surface may be treated which minimizesre-contamination and materials compatibility problems using thetechniques described herein. Furthermore, the present invention cantreat nearly all types of contamination typically found on variousprecision substrate surfaces and can produce a physicochemicallymodified surface which is necessary for subsequent manufacturingoperations such bonding, plating, coating, assembly, or for direct use.

SUMMARY OF THE INVENTION

[0019] The present invention employs various combinations of solidcryogenic carbon dioxide spray cleaning, liquid and supercritical carbondioxide immersion cleaning, atmospheric plasma, and ultraviolet/ozonecleaning. Furthermore, the present invention teaches the use of a newdry cleaning technique called electrohydrodynamic (EHD) cleaning for theremoval of nanoscopic surface contaminations. Although very attractive,these individual techniques have discrete characteristics, that isapplication and performance limitations, which prevent them individuallyfrom properly treating a substrate surface and may even damage asurface. For example, physical damage to a substrate surface may becaused by a particular technique due to excessive cleaning energiesrequired (i.e., high spray pressure, high plasma energy level, presenceof oxidizing chemistries) and excessive treatment periods required toachieve a certain surface cleanliness level. The individual technologiesemployed in combination in the present process. Moreover, many times thenative substrate surface must be chemically activated to insure goodadhesion or wetting for follow-on manufacturing processes. Finally,water reduction and pollution prevention are a major concern for theprecision device manufacturing industries.

[0020] Most conventional wet and dry combinations create significantwaste by-products, pose worker exposure dangers, and consume tremendousamounts of water resources. Much interest exists to develop alternativeprecision substrate cleaning and drying methods to replace hazardouschemicals such as organic solvents, acids and hydrogen peroxide.Technological advances in this area such as MicroelectromechanicalSystems (MEMS) and shrinking line widths and deep trenches with highaspect ratios require advanced cleaning and drying technologies.Industry utilizes or has proposed various techniques to remove plasmareacted or patterned organic photoresists (i.e., Table 1, Type A1contamination) and particles (i.e., Table 1, Type C2 and C3contamination), rinse and dry a semiconductor wafer. An example of atypical conventional and combinational cleaning technique forsemiconductor substrates follows.

[0021] With respect to cleaning wafers to remove an organic photoresistcontamination, commercial cleaning systems have been developed whichemploy ozone and water to replace dangerous or ecologically-unsafechemical processes such as sulfuric acid-hydrogen peroxide mixtures,toxic organic solvents, and amine-based cleaning agents. One suchsystem, called the SMS DI03 photoresist strip process (Legacy SystemsInc., Fremont, Calif.), uses an ozone generator and diffuser located ina tank of chilled (5 C) deionized water which is circulated into a tankcontaining the wafers. Ozone is a powerful oxidizer that is used tomineralize organic contamination. following ozone treatment, waterrinsing and drying are performed. However, complete drying of precisionsubstrates following cleaning by wet methods is limited due to hydrationof small capillaries, vias and interstices that may be present.Moreover, a lack of substrate surface selectivity can be limiting inmany applications, because the entire precision device is subjected tothe combinational cleaning method that complicates cleaning, drying andcompatibility issues. Drying methods typically employ an alcohol rinseto overcome some of these issues. For example, techniques include theuse of an isopropyl alcohol (IPA) vapor dryer, full displacement IPAdryer, and others. These IPA-type dryers often rely upon a largequantity of a solvent such as isopropyl alcohol and other volatileorganic liquids to facilitate drying of the semiconductor wafer. Anexample of such a technique is described in U.S. Pat. No. 4,911,761, andits related applications, in the name of McConnell et al. and assignedto CFM Technologies, Inc. McConnell et al. Generally describes the useof a superheated or saturated drying vapor as a drying fluid. Thissuperheated or saturated drying vapor often requires the use of largequantities of a hot volatile organic material. The superheated orsaturated drying vapor forms a thick organic vapor layer overlying therinse water to displace (e.g., plug flow) such rinse water with thedrying vapor. The thick vapor layer forms an azeotropic mixture withwater, which will condense on, wafer surfaces, and will then evaporateto dry the wafer. A limitation with this type of drying technique is itsuse of the large solvent quantity, which is hot, highly flammable, andextremely hazardous to health and the environment. Another limitationwith such a drying technique is its cost, which is often quiteexpensive. In fact, this dryer needs a vaporizer and condenser to handlethe large quantities of hot volatile organic material.

[0022] As line size becomes smaller and the complexity of precisionmanufactured devices increases, it is clearly desirable to have an alldry cleaning and surface treatment technique, including both method andapparatus, that selectively removes unwanted organic films ad particles,prevents additional particles, and does not introduce compatibilityproblems for the manufactured device. The complete selective cleaningtechnique may also include a step of drying the precision substrate,without other adverse results. A further desirable characteristicincludes reducing or possibly eliminating re-contamination of precisionsurfaces during cleaning and handling. The aforementioned conventionaltechnique fails to provide such desired features, thereby reducing theyield of good precision devices.

[0023] From the above, it is seen that a method and apparatus forcleaning and precision drying semiconductor integrated circuits that isdry, safe, easy, and reliable is desirable. There is a present need fora all dry and enabling combinational method which can produce anydesired level of cleanliness down to the nanoscopic scale, and startingwith various levels and types of contamination. Moreover, a robust andall dry cleaning method is desired to achieve the desired surface orsubstrate cleanliness and surface energy. Still moreover, a cleaningmethod is desired which optimizes the capabilities of each technique toachieve a stepwise reduction in contamination levels without causingdamage to the precision surfaces. Finally, a non-toxic andenvironmentally friendly dry cleaning method is desired to eliminatepollution, reduce hazardous waste by-product generation, reduce waterusage and eliminate worker exposure to toxic, corrosive, or carcinogeniccleaning chemicals. TABLE 4 Surface Modification Types Type A—AdhesionPromotion Type B—Hydrophilic Properties Type C—Oleophobicity andHydrohobicity Type D—Surface Friction Type B—Barrier Films TypeF—Biocompatibility

[0024] Still moreover, the aforementioned conventional cleaningapproaches do not have the capability of modifying or treating precisionsurface once cleaned. Again this is due to the inherentincompatibilities between the conventional surface cleaning andmodification techniques. Referring to Table 4 above, surfacemodification schemes may include etching away a thin layer of native andclean surface to increase wetability or to promote adhesion strengthduring subsequent manufacturing operations such as adhesive bonding.Moreover, surface modification also may include depositing small amountsof organic or inorganic molecules onto a cleaned surface to decreasefriction, invention are described in the following sections. Thisdiscussion includes an assessment of benefits and performancelimitations associated with each dry cleaning technique.

[0025] The present invention is a combinational method which enables theremoval of most levels and types of macroscopic, microscopic andnanoscopic contaminants, thick films, thin films, absorbed contaminants,interstitial residues and particles as described in Table 1 herein. Thepresent method and exemplary treatment apparatuses taught herein havebeen developed as a result of the present inventors understanding andexploitation of the relationships between the various dry cleaningmechanisms and performance profiles for plasma, dense fluid, UV/O₃ andEHD dry cleaning and surface preparation treatments. A brief discussionof each technique used in the present invention follows.

[0026] Vacuum and Atmospheric Plasma Cleaning and Modification Technique

[0027] Vacuum and atmospheric plasma cleaning uses an electricallycharged gas containing ionized atoms, electrons, highly reactive freeradicals, electrically neutral species, and ultraviolet radiation.Plasmas are produced in a multi-stage process by passing an electriccurrent through the process gas. The resulting plasma is highly reactivewith surface contaminants. Plasmas can be used in a wide range oftemperature and pressure conditions; however, cold plasmas (those withtemperatures less than 140° F. [60° C.]) are most often used forcleaning applications. Normal operating pressures for vacuum plasmacleaning processes range from 1 to 500 millitorr. Vacuum chamber plasmasmay be used to treat large surfaces and entire substrates, whereasatmospheric and enhanced capillary discharge plasmas may be usedselectively to treat only a portion of a precision substrate surface.

[0028] In general, for thick and thin film contaminations plasmacleaning can produce extremely clean surfaces in minutes. Since thecleaning medium is a gas, hidden areas of complex parts can be cleanedbetter (albeit, rather slowly) than line-of-sight processes, such ascarbon dioxide spray cleaning. Most plasma gases are selective in theircleaning ability, removing either organic contaminants or inorganiccontaminants, but not both. Therefore, gas selection and mixing iscritical. For example oxygen mixtures may be used for hydrocarboncleaning, hydrogen mixtures for oxide removal and fluorinated mixturesrecalcitrant carbonaceous residues or highly cross-linked polymers. Inaddition, some types of energetic plasma, and especially vacuum plasmas,can cause erosion of critical dimensions on metal or epoxy surfaces ifthe conditions are not carefully controlled. Moreover, long treatmenttimes are required for thick film contaminants and for complete removalof all carbonaceous residues.

[0029] Finally, following combinational surface cleaning techniquesdescribed herein, gaseous or vaporous admixtures may be injected into adense fluid-plasma process chamber under low pressure and plasma energyconditions to produce a chemically modified clean surface. For example,this surface may be processed to have a thin fluorocarbon film, a Tefloncoating, which provides a low friction abrasion barrier for the cleanedsurface. Furthermore, the cleaned surface may be activated to preparefor cell growth or protein bonding (for example an implant surface) orfor enzyme bonding (for example a platinum sensor surface). Polymericprecision devices such as medical instruments, optical elements, andother critical medical devices, once cleaned, can be treated to producehighly wettable, low permeable, clean, and sterile substrate surfacesusing the present invention.

[0030] Exemplary plasma cleaning and modification systems suitable foruse in the present invention are available from 4th State Inc., Belmont,Calif. (Vacuum Plasma) and SurFx Technologies, LLC, Los Angeles, Calif.(Atmospheric Plasma).

[0031] An aspect of the present invention is to exploit the variousplasma techniques to first chemically alter a surface contaminant orweaken adhesion forces between organic thick film contaminant andsubstrate surface—thereby changing its state so that a combinational andsecondary technique such as CO₂ spray or liquid immersion may be usedmuch more effectively and efficiently. Moreover, a low pressure plasmasurface modification treatment may immediately follow the cleaningtreatments to physicochemically modify a cleaned surface to produce aclean and modified surface which is wettable, bondable, biocompatible,or exhibits enhanced surface characteristics such as sterility,impermeability, low friction, or enhanced light reflectance. This wouldnot be possible without first using the first combinational surfacecleaning operation. Moreover, this aspect of the present invention maybe combined into a single process tool in which a precision substratesurface is cleaned and modified in a single operation.

[0032] Dense Fluid Surface Cleaning Technique

[0033] Liquefied gases and supercritical fluid cleaning technologies(i.e., Dense Fluids) use the enhanced solvent characteristics ofcompounds that are heated and pressurized to near or above their uniquecritical points. Such fluids have the solvent power of a conventionalliquid cleaning agents and the transport properties of a gas. Carbondioxide is the most-frequently used dense fluid, due to the lowtemperature and pressure of its critical point. Dense phase carbondioxide can effectively remove oils, lubricants, and other organiccontaminants. A typical dense fluid cleaning process consists of threesteps: gas pressurizing and heating, extraction of contaminant, andgas-contaminant separation. Once the extracting fluid has cooled and/orde-pressurized below its critical point, the solubility of the fluiddecreases, the contaminants can be readily removed, and more than 90percent of the gas can be recovered for reuse. Cleaning with densefluids is well suited for many precision substrate surface cleaningapplications because of its compatibility with a wide variety ofmaterials and because no solvent residue remains on component surfacesafter cleaning. However, dense fluids may not be compatible with someelastomers (such as Viton) and causes swelling and cracking in somepolymer materials. Additional limitations of dense fluid cleaninginclude the difficulty in removing high molecular weight hydrocarbons orhighly cross linked organic molecules, and the possibility of damagingdelicate components as a result of the high system pressure.

[0034] An aspect of the present invention is to exploit the variousplasma techniques to first chemically alter a surface contaminant orweaken adhesion forces between said contaminant and substratesurface—thereby changing its state so that a combinational and secondarytechnique such as liquid CO₂ immersion may be used much more effectivelyand efficiently. Moreover, plasma treatment may immediately follow thesecondary treatment to chemically modify a clean surface. This would notbe possible without first using the combinational cleaning operation.

[0035] Alternatively, dense fluids may be expanded or condensed to formsolid sprays that may be used as physical cleaning agents—cryogenicdense fluid sprays. Solid carbon dioxide (CO₂) and argon (Ar) ice spraycleaning processes may be used to remove organic contaminants andparticulates by an impact/flushing method. Of the two distinct CO₂cleaning processes, CO₂ snow and CO₂ pellets, CO₂ snow is more suitedfor typical precision particle and thin film residue cleaningapplications and pellets are more suited for gross particle and thickfilm contaminant removal. CO₂ snow is formed when liquid carbon dioxideis allowed to rapidly expand through a nozzle. This creates solidparticles of CO₂ (i.e., snow) entrained in a stream of pressurized CO₂gas. Furthermore, CO₂ snow may be compressed into larger pellets andused as a more aggressive treatment media. Argon ice spray cleaning issimilar to snow cleaning, with an argon ice particle spray formedthrough the combination of argon gas with liquid nitrogen.

[0036] The solid particles contact contaminant particles on thesubstrate and remove them through a cryo-kinetic and momentum transferprocess as well as thin film solubilization. Following impact, the solidparticles then transform into a gas (sublime) and thus do not add anyvolume to the waste stream.

[0037] CO₂ snow and Ar spray cleaning are non-abrasive processes and aretypically used as a final clean following other more aggressivepre-cleaning steps herein, for example a plasma or CO₂ pellet spraypre-treatment technique. Snow has some ability to remove molecular filmsof organic contaminants but cannot remove heavy amounts of organiccontaminants and cleaning is typically restricted to line-of-sight.Argon ice cleaning is even more restricted to fine particlecontamination removal. Other potential limitations of dielectric solidspray cleaning include thermal shock concerns and condensation build-up,which can inhibit cleaning. These latter two drawbacks can be overcomeby proper design of a cleaning method as addressed using the presentinvention, which limits exposure of a substrate to long treatmentperiods or to excessively high spray pressures.

[0038] In still another example of a dense fluid treatment step, densefluid sprays comprising pressurized and superheated carbon dioxide andtrace amounts of water vapor (steam), the subject of a PCT applicationby the present inventor, have been found by the present inventor to bevery effective for removing tenacious contaminants such as waxy orgrainy buffing and polishing compounds. This type of dry dense fluidsteam spray is suitable as a gross pre-clean prior to snow spraycleaning operations above.

[0039] Dense fluid cleaning systems, including gas, solid, liquid andsupercritical fluid systems, suitable for use in the present inventionare available from The Defiex Corporation, Valencia, Calif.

[0040] An aspect of the present invention is exploit the unique solvencyand surface scouring characteristics provided by the aforementioneddense fluid immersion and spray cleaning techniques. However, to usethis technique effectively, it has been found that the surfacecontaminant must be first treated to eliminate or reduce contaminantcharacteristics such as dryness, lack of solubility, organic crosslinking, tackiness, and thickness.

[0041] The above dry cleaning techniques may be used in variouscombinations to chemically and physically treat a precision substratesurface to remove thick and thin film contaminants efficiently down tothe 50 angstrom level and particle residues to the 0.2 micronlevel—considered microscopic level cleaning. However, to continuetreating to below these levels, which is to the nanoscopic level,additional combinational methods must be employed. These are describedin the following sections.

[0042] Electrohydrodynamic Surface Cleaning Technique

[0043] Electrohydrodynamic (EHD) cleaning is a vacuum cleaning techniquethat utilizes microscopic and energetic cluster beams to removesub-micron residues adhering to a native substrate surface.Electrostatically charged micro droplets or clusters having apre-determined chemistry and composition, velocity, energy and size aredirected at a precision substrate surface under vacuum conditions. Microclusters are extremely effective for removal of sub-micron levelcontaminations, without leaving a residue, but are highly directionaland can be easily blocked by thick films or large particles (i.e., >5microns) and complex topography present on a substrate surface. Forexample, microscopic mirrors on an optical wafer will occlude the microcluster beam. An EHD cleaning system suitable for use in the presentinvention is available from Phrasor Scientific, Duarte Calif.

[0044] An aspect of the present invention is to exploit the combinationsof plasma and dense fluid above to first chemically clean and modify aprecision substrate surface in preparation for nano-scale cleaningtreatments using EHD. The EHD technique is enabled by the combinationaltechniques that precede it.

[0045] Ultraviolet Light with Ozone Surface Cleaning Technique

[0046] The ultraviolet/ozone (UV/O₃) cleaning process involves theexposure of a contaminated precision surface to ultraviolet light in thepresence of ozone. Cleaning occurs when contaminant molecules areexcited or dissociated by the absorption of short-length UV light. Atthe same time, the ozone breaks down into atomic oxygen, which thenreacts with the excited contaminant molecules and free radicals to formsimpler, volatile molecules, such as carbon dioxide and water vapor.

[0047] UV/O₃ cleaning produces surfaces that meet critical cleanlinessrequirements. The UV/O₃ cleaning process has been used successfully toremove very thin organic films from a number of different surfaces inprecision cleaning applications. This process is relatively inexpensiveto set up and operate and, since it has no moving parts, is easy tomaintain. However, UV/O₃ cleaning does not remove inorganic contaminantsor particulates. Moreover, UV/O₃ process has a line-of-sight cleaninglimitation and the possibility of staining, discoloration, or corrosionof surfaces that can result from improper wavelengths or exposure times.Therefore, this step is used as a polishing step following the above drycleaning combinational techniques. UV/O₃ cleaning systems suitable foruse in the present invention are available from Jelight Company, Inc.,Irvine, Calif.

[0048] An aspect of the present invention is to first use a plasma anddense fluid combinational cleaning technique to first chemically cleanand modify a precision substrate surface. However, the aforementionedchemically and physically pretreated substrate surface still containsmolecular levels of contaminants, both films and residues. For example,the clean and modified surface can then be exposed to a UV/O₃ treatmentfor a few seconds, which rapidly produces a molecularly clean surfaceand enhances the outgassing of absorbed films and gases from a substratesurface. This would not be possible, without first using the firstcombinational cleaning operation described herein.

[0049] Thus, the alternative combination of dry surface cleaning andmodification technologies discussed above and used in the presentinvention are very attractive but have limitations due to varying levelsof cleaning performance, line-of-sight effectiveness, and potentialdamage (i.e., plasma etching) to substrate surfaces if contacted for anextended treatment period or if used at excessive energy levels.;However, if used in certain combinations, an instant cleaning method maybe established for removing a variety of contaminants from precisionsubstrate surfaces based on the nature of and interaction betweencontamination and surfaces. The nature of the various contaminants andsubstrates; the contaminant-substrate and contaminant-contaminantadhesion forces present must be fully understood. Furthermore, thevarious interactions between the cleaning method and the substrate andsubstrate features present thereon must be understood. Once all of theseinterrelationships and discrete cleaning parameters are understood, aninstant and enabling relationship may be established which selectivelyremoves a contaminant or group of contaminants from a substrate, inpre-determined and discrete steps, without damaging the varioussubstrate features which may be present, for example patterned resists,microvias, microstructures, and beneficial coatings. Moreover, thepresent invention uniquely and easily lends itself to being performed ina single process chamber or integration within a staged or in-linecluster tool. This is beneficial since it reduces re-contamination ofprecision substrate surfaces during handling.

[0050] An aspect of the present invention is to ascertain and apply aninstant and enabling dry surface treatment combination which eliminateswet processing and rinsing and drying steps, increases cleaning andmodification tool productivity, decreases equipment cost, and size,reduces pollution, and improves substrate cleanliness, quality andyield.

[0051] The present invention illustrates a method in which an instantenabling, dry, and selective cleaning combination is established usingfour dry techniques described above; plasma, dense fluids,electrohydrodynamic cleaning and UV/Ozone cleaning. Using two or more ofthese techniques sequentially or simultaneously as an instantcombination, virtually any type of contamination may be effectively andselectively removed from a substrate without damaging the precisionsubstrate surface.

[0052] The present invention may be used in the field of precisionmanufactured and assembled devices and precision test apparatus such aswafers, dies, CMOS image sensors, fiber optic connectors, opticalfibers, optical benches, optics, IC test socket pads, flexible polyimidegold circuits, PCB rework, lead frame bond pads, and photodiodes, amongmany others. The invention is illustrated in various examples asfollows.

[0053] 1. Patterned MEMS die—selective removal of residues, particlesand absorbed gases from complex topography;

[0054] 2. Polyimide gold circuit film—selective removal of a protectiveorganic coating from gold metallized layers in preparation for platinumelectroplating;

[0055] 3. Optical fiber—selective removal of acrylate polymer,fingerprints, and particles from stripped or spliced optical fibersurfaces and surface energy increase in preparation for bonding;

[0056] 4. Optical filter—selective removal of a proprietary organic filmand particles and surface modification in preparation for coating; and

[0057] 5. Optical connector—selective removal of polishing residues andfilms from an end face in preparation for use; and

[0058] 6. PCB rework—selective removal of out-of-spec electroniccomponent, preparation of bond pad area, and bonding of new speccomponent; and

[0059] 7. IC Test Socket Cleaning—selective surface re-conditioning ofIC socket test pads.

[0060] However it will be recognized that the invention has a much widerrange of applicability. Merely by way of example, the invention can alsobe applied to selective cleaning of disk drive read-write heads, dicedwafers, image sensors, optical sensors, implantable medical devices,lead frames, LCDs, OLEDs, photodiodes, and many other precision devicesand surfaces.

[0061] Moreover, medical substrates such as boroscopes, polyestergrafts, polyurethane blood filters may be treated using the presentinvention to remove residues, particles, biological contaminant and maybe treated using plasma techniques to improve wetability andbiocompatibility. Still moreover, the plasma-dense fluidcleaning-modification combination may be used to clean and treatcommercial textiles and fabrics to remove complex surface soils and tobrighten fabric fiber surfaces, respectively.

[0062] The present invention provides a safe, robust, and selectivemethod and apparatus to treat a precision substrate surface using anenabling combination of atmospheric plasma, dense fluids, UV/O₃, andelectrohydrodynamic (EHD) cleaning techniques, which when used invarious combinations described herein, an entire spectrum of surfaceresidue and particle cleaning performance to the nanometer level, and isbetter understood by reference to the following figures and detaileddiscussion that follows.

[0063]FIG. 1—Performance profiles for the exemplary dry cleaningtechniques; Plasma, Dense Fluids, EHD, and UV/O₃.

[0064]FIG. 2—Exemplary combinational dry cleaning approach formulti-layered contamination on a substrate surface.

[0065]FIG. 3—Exemplary flow diagram showing the various instant drycleaning methods possible using the present invention.

[0066]FIG. 4—Exemplary dry cleaning method options matrix thatcorrelates the contaminants, substrate surface, and enabling drycleaning techniques.

[0067]FIGS. 5a and 5 b—Exemplary photomicrographs at 500× magnificationshowing before and after surface cleaning large and small particlesusing vacuum plasma and dense fluid spray cleaning treatments,respectively.

[0068]FIGS. 6a and 6 b—Exemplary photomicrographs at 500× magnificationshowing before and after surface cleaning of large and small particlesusing an EHD spray.

[0069]FIGS. 7a and 7 b—Exemplary photomicrographs at 2500× magnificationshowing before and after surface cleaning of large and small particlesusing an EHD spray.

[0070]FIGS. 8a and 8 b—Exemplary photomicrographs at 2500× magnificationshowing before and after surface cleaning of small particles using anEHD spray.

[0071]FIG. 9—Exemplary polyimide-gold flexible circuit substrateselective cleaning application.

[0072]FIG. 10—Exemplary in-line surface treatment apparatus for thesubstrate of FIG. 9.

[0073]FIG. 11—Exemplary printed circuit board substrate selectivecleaning application.

[0074]FIG. 12—Exemplary cluster cleaning and assembly apparatus for thesubstrate of FIG. 11.

[0075]FIG. 13—Exemplary IC socket test substrate selective surfacecleaning application.

[0076]FIG. 14—Exemplary optical device selective surface cleaningapplication.

[0077]FIG. 15—Exemplary fiber optic connector selective surface cleaningapplication.

[0078]FIG. 16—Exemplary MEMS wafer selective surface cleaningapplication.

[0079]FIG. 17—Exemplary combinational cluster cleaning tool forperforming sequential treatments of a precision substrate surface.

DETAILED DESCRIPTION OF THE INVENTION

[0080]FIG. 1 shows performance profiles for the exemplary dry cleaningtechniques; Plasma, Dense Fluids, EHD, and UV/O₃ used in the presentinvention. Referring to FIG. 1, performance profiles for plasma (2),dense fluid (4), EHD (6), and UV/O₃ (8) dry cleaning and surfacepreparation treatments are represented a Gaussian distribution curves.The performance profiles represent generalized upper and lower limits ofcleaning efficiency for a certain class of contaminants. A generalizedboundary condition (10) exists which demarks the transition from macroand microscopic layers (12) to nanoscopic layers (14) of contamination,and to the rough and porous native substrate surface. Furthermore, theindividual treatment groups bisected by the boundary condition (10) maychange in sequence, or may be used selectively. For example, plasma (2)and dense fluids (4) are used in combination with the present inventionto address macroscopic and microscopic contaminations, as well assurface modification treatments (Table 4). Following plasma and densefluid surface pre-treatments, EHD (6) and UV/O₃ (8) may be usedselectively to address nanoscopic and molecular contamination concerns,respectively.

[0081] As shown in FIG. 1, plasma cleaning provides various Table 3,Class B and C cleaning energies and dense fluids provide various Table3, Class A, B, and C cleaning energies. Thus these two combinationaltechniques can provide a range of cleaning energies suitable for mostTable 1, Class A and B contaminations and can be used efficiently invarious forms and combinations to remove nearly 100% of particles assmall as 0.5 micrometer and residues down to the molecular level. Atthis point (10), nanoscopic contaminations in the form of nano-sizedparticles, molecular films, and outgassing compounds are exposed on asubstrate surface. More efficient dry cleaning techniques must beemployed for nanoscopic and molecular contaminants, but without apreceding surface treatment, these contaminants remain encapsulated inthicker contaminating films and larger particles. Light obscuration,particle hideout, capillary forces, Van der Waals force, and stictionshield or hold these low level contaminations to the surface and preventvarious cleaning energies from effectively accessing and removing thesenanoscopic contaminations. EHD cleaning provides Table 3, Class A1cleaning energy required to remove very small particles and UV/O₃provides Table 3, Class C2 cleaning energy to remove absorbed gases andvapors from the native surface. As shown in FIG. 1, EHD and UV/O₃combinational cleaning provides a range of effective cleaningperformance in a range from 0.5 micrometers down to 0.01 micrometers,including the removal of molecular contaminants.

[0082]FIG. 2 shows the exemplary combination of dry cleaning techniques,conditions of use, and sequencing to remove various layers ofcontamination. Referring to FIG. 2, a contaminated surface may becharacterized as follows; thick polymeric films (16) can be on the orderof several microns thick, followed by viscous oily films and largeparticles (18), followed by very small particles (20), and finallymonolayer films and outgassing contaminants (22) on or within the poresof a rough native substrate surface (24). At the surface level,microscopic pores and capillaries (26) present on the substrate surface(24) entrap very small particles and absorbed molecules and vapors.

[0083] Thus as characterized in FIG. 2, it can be seen that each uppercontaminant layer encapsulates the layer below it. Moreover, thephysicochemistry of each layer may be different in terms of quantity,physicality, and bonding energies. The present invention utilizes theaforementioned dry cleaning techniques in various combinations and underlimited contact periods in an enabling sequence to remove the variouslayers shown in FIG. 2 with increasing precision. For example, as shownin FIG. 2, atmospheric plasma (28) may be used very efficiently andselectively in a 2 minute exposure period to reduce a polymericcontaminant layer (16), which lowers bonding energy and increasessurface area. Following this, a dense fluid spray (30) is usedselectively to remove the reduced contaminants and particles freed bythe plasma treatment (28). If desired and required, EHD (32) may be usedto remove nanoscopic residues freed by the dense fluid treatment (30).Finally, as a polishing step, UV/O₃ (34) may be used remove or flash offthe remaining monolayers of absorbed gases and vapors.

[0084] Furthermore, following plasma and dense fluid combinationalsurface cleaning treatments described above, a native and clean surfaceis exposed. A second plasma treatment (36) may be again selectivelyapplied, although using the same dense fluid-plasma cleaning treatmentdevice, to the substrate surface to modify a portion of the substratesurface, for example the removal of oxides (i.e., Table 1, Type B2contaminant) using a hydrogen plasma gas mixture, in preparation forbonding (i.e., Table 4, Type A modification). Moreover, as discussedherein, the additional of special admixture gases during a plasmatreatment will impart beneficial surface properties to a properlycleaned and activated surface such as increased impermeability orpermeability, low friction, biocompatibility, brightness or otherphysicochemical surface features. Thus the techniques are arranged andapplied in a specific and enabling sequence to achieve a desired levelof cleanliness and surface modification.

[0085]FIG. 3 is an exemplary flow diagram showing the various exemplaryinstant dry cleaning (Methods 1-7) and cleaning-modification (Method 8)methods possible using the present invention. Referring to FIG. 3, thefour dry techniques employed produce several possible instant precisionsubstrate surface cleaning and modification methods as shown in Table 5.TABLE 5 Instant Surface Cleaning Methods Exemplary Instant Method 1:Step 1—Plasma Cleaning (38) Step 2—Dense Fluid Cleaning (40) Step 3—EHDCleaning (42) Step 4—UV/O₃ Cleaning (44) Exemplary Instant Method 2:Step 1—Plasma Cleaning (38) Step 2—Dense Fluid Cleaning (40) Step3—Plasma Surface Modification (38) Exemplary Instant Method 3: Step1—Plasma Cleaning (38) Step 2—Dense Fluid Cleaning (40) Step 3—UV/O₃Cleaning (44) Exemplary Instant Method 4: Step 1—Plasma Cleaning (38)Step 2—Dense Fluid Cleaning (40) Exemplary Instant Method 5: Step1—Dense Fluid Cleaning (40) Step 2—Plasma Cleaning (38) Step 3—DenseFluid Cleaning (40) Exemplary Instant Method 6: Step 1—Dense FluidCleaning (40) Step 2—EHD Cleaning (42) Exemplary Instant Method 7: Step1—Dense Fluid Cleaning (40) Step 2—UV/O₃ Cleaning (44) Exemplary InstantMethod 8: Step 1—Plasma Cleaning (38) Step 2—Dense Fluid Cleaning (40)Step 3—Plasma Modification (38)

[0086] A particular instant dry cleaning method is chosen for aparticular surface cleaning application and is based on the type ofsubstrate, contaminants, cleaning time, and desired level ofcleanliness.

[0087]FIG. 4 is a dry cleaning method options matrix that correlates thecontaminants, substrate surface, and enabling dry cleaning methodsdescribed herein and in Table 5. Moreover, FIG. 4 provides fourexemplary precision cleaning applications and shows the optimal instantmethod from Table 5 for each application.

[0088] Referring to FIG. 4, the exemplary contaminants described inTable 1 herein form the column (46) and the exemplary substrate surfacesof Table 2 herein form the row (48) of a cleaning options matrix (50).The cleaning options matrix (50) comprises the various cleaningtechniques suitable to address the specific contaminant (46) on aparticular substrate surface (48). From this matrix, an instant surfacecleaning and treatment method is created based on the real-worldcontaminant-substrate surface application. Referring to FIG. 4, surfacetreatment application examples (52) comprise the following; the topsideof an IC wafer to remove RIE etch residues and particles in preparationof a resist coating (54), the interior of a CMOS image sensor to removestains, solder flux, and particles in preparation for hermetic sealing(56), the gold bonding pads on an organic polyimide flexible circuit toremove resist and particles in preparation for platinum electroplating(58), and the exposed optical fiber to remove finger oils, particles andstripping residues in preparation for adhesive bonding (60).

[0089] As shown in FIG. 4, an instant method (62) as described in FIG. 3above was used which encompassed the nature and level of contaminationpresent and to meet the cleanliness objectives for each cleaningapplication. For the IC wafer application, exemplary instant method 1described above in Table 5 and FIG. 3 (64) met the cleaning objective.For the CMOS image sensor application, exemplary instant method 3described above in Table 5 and FIG. 3 (66) met the cleaning objective.For the polyimide-gold pad cleaning application, exemplary instantmethod 4 described above in Table 5 and FIG. 3 (68) met the cleaningobjective. Finally, for the optical fiber cleaning application,exemplary instant method 5 described above in Table 5 and FIG. 3 (70)met the cleaning objective.

[0090]FIG. 5a is an exemplary photomicrograph at 500× magnificationshowing a surface that has been treated with a vacuum plasma at 100mTorr with a nitrogen-oxygen atmosphere. As can be seen in the figure,plasma treatment does not remove particles on a surface. Because plasmaenergy is predominantly oxidative, the process is rather slow for largeand inorganic particle contaminations. Various sized particles can beseen in the figure including a large 10 micrometer sized particle (72),numerous 1 micron particles (74), and a 0.5 micron particle (76). Anextended plasma treatment could eventually remove these particles fromthe surface through aggressive oxidative destruction, but an extendedtreatment is inefficient and, more importantly, would also attack anddamage the native substrate surface and any delicate features such asmicromotors and microoptics. Thus a short plasma treatment is beneficialif selectively used to remove thick and thin organic film contaminationin preparation for a follow-on thin film and particle cleaningtechniques.

[0091] Referring to FIG. 5b, a dense fluid spray was used to clean aplasma treated surface as in FIG. 5a. As can been seen in FIG. 5b, adense fluid spray treatment produces a particle clean surface (78) at500× magnification. Also seen in the figure is the rough surfacetopography showing the presence of small pits (80), ridges (82), andvalleys (84). Surface features such as these, and as depictedgraphically in FIG. 2, (24) and (26), present a major challenge toconventional surface cleaning approaches, including dense fluid sprays.These surface features hide small particles and residues from theimpacting cleaning media such as snow. A dense fluid spray is generallyeffective for removing small particles present in on the ridges (82) andvalleys (84), however the pits (80) represent a significant challenge.Increasing the dense fluid spray duration may dislodge small particlesand residues trapped within these surface depressions, however thiswould require an extended surface treatment or increased spray pressureand risk the possible freezing or encapsulating the small particleswithin the pit or possibly damaging delicate surface features with anincreased duration or impact pressure. Thus, similar to vacuum plasmatreatment, dense fluid sprays may be used selectively herein to removesurface thin films and particles in preparation for additional surfacetreatments, such as a follow-on plasma surface modification, or possiblya nanoscopic residue removal using more selective substrate treatmenttechniques.

[0092]FIGS. 6a and 6 b are exemplary photomicrographs at 500×magnification showing before and after surface cleaning a particledebris field using an EHD spray, respectively. Referring to FIG. 6a, amassive number of inorganic particles are present on the surface,including innumerable sub-micron particles still invisible at thismagnification, hundreds of 0.5 to 5 micron particles (86), and up to alarge 20 micron particle (88). Referring to FIG. 6b, following cleaningusing EHD treatment it can be seen that all the smaller particlesvisible in FIG. 6a have been removed. However, many 1 micron particles(90) and all of the larger particles; 5 micron (92), 10 micron (94), anda large 20 micron particle (96) remain. Moreover, large particles (98)can be seen hiding within a large surface groove (100). As is clearlydemonstrated by comparing FIGS. 6a and 6 b, EHD treatment is ineffectivefor removing large particles from a substrate surface and very selectivefor removing small particle contaminations. Thus the exemplary plasmaand dense fluid surface treatment techniques depicted in FIGS. 5a and 5b enable a much more selective process such as EHD. Moreover, and notshown here, a more selective technique such UV/O₃ would be similarlyenabled by the aforementioned surface treatment techniques because itsuffers from same performance limitations described herein such asresidue obscuration effects. This phenomenon is exemplified in thediscussion that follows using FIGS. 7a and 7 b.

[0093]FIGS. 7a and 7 b give exemplary photomicrographs at 500× and 2500×magnification, respectively, showing the EHD cleaned surface of FIG. 6b.As can be seen in FIG. 7a, the surface debris field contains numerousand variously sized particles having diameters generally greater than0.5 microns, and in particular, a very large 20 micron particle (102) inthe center of the figure. A close-up of the area bounded by therectangle (104) is shown in FIG. 7b. Examination of this EHD cleanedsurface at 2500× clearly shows the phenomenon of spray obscuration,which causes an impingement spray such as EHD micro cluster sprays to beeffectively blocked by larger particles and residues present on asurface. The large particle (104) shields a swath of small particles(106) at its base, having diameters of 0.2 microns and smaller. However,sub-micron particles such as those (106) hidden at the base of the largeparticle (104) are not present in exposed regions of the surface (108).

[0094] UV/O₃ cleaning is even more selective with respect to particlecleaning and line-of-sight cleaning limitations. Since only UV lightoxidation cleaning mechanisms are involved, substrate surfaces presentunder a large particle such as shown in FIG. 7b would not be exposed toUV light energy and thus not cleaned. Thus it is extremely beneficial tofirst clear the surface of thick films and large particles to enableselective surface cleaning process steps EHD and UV/O₃ cleaning. This isillustrated in FIGS. 8a and 8 b below.

[0095]FIGS. 8a and 8 b are exemplary photomicrographs at 2500×magnification showing before and after surface cleaning of sub-micronparticles using an EHD spray, respectively. As shown in FIG. 8a,numerous sub-micron particles are present on an exposed and relativelyplanar surface (110) as well as several particles (112) contained withina small surface trench (114). The surface, once pre-cleaned using aplasma and dense fluid to remove thick film residues and largeparticles, is properly prepared for a much more selective treatmentusing EHD. As shown in FIG. 8b, the surface (116) and trench (118) arefree of sub-micron particles following EHD treatment. Moreover, thissurface may be efficiently and effectively treated with UV/O₃ cleaningto remove molecular and outgassing contaminants present on the EHDcleaned surface because there are no shielding particles and residuespresent.

[0096] Having thus described the particular cleaning performancelimitations and enabling and overlapping benefits of using the plasma,dense fluids and EHD cleaning treatments herein, following is adiscussion of exemplary precision substrate treatment applications andapparatuses using the present dry cleaning method.

[0097]FIG. 9 is a graphic representing a portion of a polyimide-goldflexible circuit substrate that has been separated from a roll ofmaterial containing hundreds of these substrates (a 3M product). Theexemplary substrate is predominantly organic having a gold circuit trace(120) and gold bonding pad (122) encapsulated between two sheets of thinpolyimide polymer (124). Moreover, the gold bonding pads (122) contain athin film of organic resist (126) on their surfaces. The presence ofthis contaminant (126) prevents the deposition of platinum onto the goldpad (126) surface. Therefore it must be removed prior toelectrodeposition. In the example illustrated here; a flexible polyimidefilm (124) with electrical circuit tracing (120) have gold bonding pads(126) is a Table 2, Type B1 substrate having a portion thereoncontaining a thin film of cured resist coating which is a Table 1, TypeA1 contaminant. The precision substrate surface thus described comprisesapproximately 98% organic film (polyimide) and 2% inorganics (gold).Without surface treatment, the gold pads cannot be electroplated withplatinum.

[0098] A conventional surface treatment procedure for cleaning thissubstrate involved cutting from the roll stock, discrete precisionsubstrate portions. Each substrate portion is then immersed in a N₂/O₂vacuum plasma (200 watts/100 mTorr) for 5 minutes. It was found thatfollowing plasma treatment of the discrete portions, the bond pads werestill heavily contaminated with organic plasma etch residues, a Table 1,Type B1 contaminant. As such various mineral acid wash and water rinsecycles were used to remove residual “plasma contamination”. It wasdetermined that vacuum plasma treatment produces a Table 1, Type B1contamination as a by-product, which is probably caused by treatingpredominantly organic substrates such as polyimide in a high energyenvironment such as vacuum plasma. A nitrogen gas spray was used to drythe plasma and acid treated surfaces. A final plasma treatment was thenused to “polish” the treated surfaces. However, the multi-steppedconventional technique thus described still produces variable surfacetreatment quality due to plasma residue formation. This is manifested inthe cleaned product as sporadic and porous electrodeposits of platinumon the gold pads (122).

[0099] Using the present invention, It was determined that if a firstand single plasma treatment is immediately followed by a short andselective dense fluid spray (treating only the gold pad surfaces), theprecision substrate surfaces could be plated with platinum, therebyeliminating the corrosive acid washes and polluting rinse steps, andrepetitive and re-contaminating plasma treatment step. Furthermore, itwas determined that an atmospheric plasma using He/O₂ for 2 minutes,followed by a 2 second snow spray, could also be used to selectivelytreat the gold pads (122) for platinum electroplating. Thus, usingatmospheric plasma allows for the treatment of an entire roll of theexemplary precision substrates without having to cut discrete substratesfrom a roll of material. This makes the new surface treatment processmuch more efficient than the old method.

[0100] Thus the present example is another illustration of the enablingcombination of plasma and dense fluids. A short 1 to 5 minute selectiveexposure of a precision substrate surface to an atmospheric oxygenplasma, followed by a 1 to 5 second spray of snow particles produces aprecision clean surface which can be reproducibly electroplated withplatinum. The present method reduces time, minimizes process steps,eliminates pollution, and improves surface cleaning quality. Moreover,the new method enables improved automation and in-line surfaceinspection, which are not easily done using conventional wet and drycleaning combinations discussed herein. For example, a plasma-densefluid cleaned surface may be immediately examined following treatmentusing an in-line surface inspection technique such as opticallystimulated electron emission (OSEE) as a quality control step. This isillustrated in the discussion that follows using FIG. 10.

[0101]FIG. 10 shows an exemplary in-line reel-to-reel surface treatmentand inspection method and apparatus for the substrate discussed in FIG.9. The exemplary polyimide-gold substrate (128) is supplied from 3MCompany on a roll (130). This roll (130) contains hundreds of discreteprecision substrates graphically depicted in FIG. 9. Construction of anin-line selective cleaning and inspection system using the presentinvention is described as follows. A machine is constructed using areel-to-reel device comprising the roll of reeled source material (130)which is fed through a slotted mounting fixture (132) which presents aportion (e.g., exposed gold pads) of the precision substrate surface toa first treatment comprising an atmospheric plasma treatment device(134) which is directed (136) at said portion of said substrate surface,immediately followed by a second selective surface treatment comprisinga dense fluid snow spray (138) which is directed (140) at the samesubstrate surfaces treated by the first treatment step. Optionally, thetreated substrate surface may be inspected for residual organic resistresidues using an optically stimulated electron emission analysis probe(142) which is directed (144) at the same substrate surface treatedusing the plasma and dense fluid sprays.

[0102] Again referring to FIG. 10, the rolled material (130) may be fedcontinuously or indexed (stop and go) as indicated by the arrow (146) topresent the portion (128) of the roll material (130) requiring a surfacetreatment and optional inspection. The treated substrates (148) arerolled onto a take-up reel (150) and a clean interleaf barrier film(152) from a supply reel (154) may be rolled up with the treatedsubstrates to protect treated surfaces (148) from being recontaminated.Also as shown in FIG. 10, an in-line nitrogen gas ionizer (156) may beused to deionize treated substrate surfaces to prevent electrostaticcharge attraction of atmospheric contaminants during handling andstorage.

[0103] In another example application shown in FIG. 11, an electronicprinted circuit board substrate (158), containing many electroniccomponents (160) requires selective substrate surface cleaning. In thisapplication, a discrete electronic component, in this example a 0201chip resistor (162), must be first removed, and the underlying surfacemust be cleaned and prepared for placement of a new component. Theportion of the substrate surface to be cleaned and treated isrepresented by the circle (164) and discussed more fully below. Theexemplary process is described as follows. The malfunctioning chipresistor (162) contains a silicone conformal coating (166), both ofwhich are removed together using a thermal de-bonding technique (168),for example an infrared laser. Following thermal de-bond, the underlyingsubstrate surface (170) is exposed and contains residual siliconeconformal coating particles and soldering flux residues (172) on thesurface (170) and on the tin bonding pads (174). A dense fluid spray(176) is used to remove residual silicone coating and flux residues fromthe immediate vicinity (178) and on the exposed surfaces of the tinbonding pads (174) to produce particle and residue free bonding padsurfaces (180). However, the cleaned bonding pad surfaces (180) stillcontain a thin film of oxide, which must be removed to provide properwetting and good adhesion during the subsequent new component solderingoperation. An atmospheric plasma surface treatment using anArgon-Hydrogen gas mixture (182) is employed to remove oxidecontamination from, the cleaned bond pads (180) to produce a residueclean and oxide free bonding pad surface (184). Finally, the cleaned andtreated pad surface (184) is ready for bonding the new electroniccomponent. At this point, the surface may be optionally inspected usingOSEE as above. A new electronic component (186) is thermally bonded(188) to the clean and treated bond pads (184) using an infrared laserand is coated with a small quantity of UV-curable silicone conformalcoating (190), which may then be cured using a UV lamp.

[0104] Thus the present example illustrates a dry and very selectivesurface cleaning, treatment and inspection method which eliminates theneed for treating the entire substrate (158) using conventional wetsurface cleaning techniques such as aqueous immersion cleaning, acidoxide removal techniques, water rinsing, and hot air drying. Moreover,the present surface treatment method may be directly integrated into asoldering rework tool, producing a much more efficient production tooland process. This capability is illustrated in the following discussionusing FIG. 12.

[0105]FIG. 12 is an exemplary cluster cleaning and assembly apparatusfor performing the method described in FIG. 11. As shown in FIG. 12, acluster cleaning and rework tool may comprise a hexagonal workstation(192), having at its center robotic transfer robotics (194). Surroundingthe substrate transfer robotics (194), five selective treatment modulesmay be positioned as follows; a thermal de-bond module (196), a surfacetreatment module (198), a bonding module (200), a conformal coatingmodule (202), and UV curing module (204). Integrated with the exemplaryworkstation (192) is incoming substrate conveyor (206) and an outgoingprocessed substrate conveyor (208). Having thus described the basiccomponents for a cleaning and rework tool, following is a description ofthe operation of such a tool.

[0106] Referring to FIG. 12, an un-processed electronic board substrate(158) having a malfunctioning or out-of-spec electronic device (162)mounted thereon is conveyed (210) into the workstation (192) using anincoming conveyor (206). A substrate transfer robot (194) moves theelectronic board substrate (158) through a series of rework andsubstrate treatment steps as described in FIG. 11. The electronic board(158) is moved counterclockwise through the workstation (192) asfollows; to the thermal de-bond module (196), to the dense fluid-plasmasurface treatment module (198), to the thermal bonding module (200), tothe coating module (202), and to the UV curing module (204). Finally,the processed substrate (212) containing a new electronic device (186)and conformal coating (190) is conveyed (214) from the workstation (192)using an outgoing conveyor (208).

[0107] Having thus described in detail two exemplary precision substratetreatment applications and apparatuses using the present dry cleaningmethod, the following discussion provide additional and more generalexamples of precision substrate surface applications for the presentinvention using FIGS. 13, 14, 15 and 16.

[0108]FIG. 13 is an exemplary substrate comprising an IC socket testapparatus. As shown in FIG. 13, an IC socket test apparatus contains anarray of gold plated test pads (216) that are affixed to a base (218),and to which is connected to a circuit test apparatus (not shown). Adevice such as a BGA chip containing a similar pattern and number ofsockets (not shown) is contacted to the topside surface (220) of the ICtest pads (216). Following this, the IC test socket apparatus tests theBGA for electrical or logic performance. This operation is performedthousands of times in production and requires periodic cleaning toremove oxides, particles and other contaminants that build-up over time.Cleaning of the contacts becomes necessary because contact resistanceincreases significantly as contaminating residues levels increase,producing false signals or misinterpretation of test results. As such,the present invention, and specifically the atmospheric plasma-densefluid spray method described herein using a nitrogen-hydrogen plasma gasmixture followed by a dense fluid spray treatment will selectively cleanand reconditioning of the gold contacting surfaces of an IC test socketsubstrate.

[0109]FIG. 14 is an exemplary selective substrate cleaning applicationcomprising an optical bench. An optical bench is assembled using ahousing (222), into which are assembled various optics, electronics andmounting fixtures. For example an optical cable (224) may be stripped toexpose a bare optical fiber, which is bonded to a v-groove blockassembly (226) and mated to a photodiode device (228). The exemplaryoptical bench thus described will convert light signals traveling downthe optical fiber into electrical signals. The topside surface (230) ofthe v-groove block assembly (226) and exposed fiber (232) must becleaned of stripping debris and other contaminations to provide forproper adhesive bonding of the fiber (232) with the photodiode assembly(228). The device as described is very difficult if not impossible toclean using conventional cleaning methods. The present invention, andspecifically the atmospheric plasma-dense fluid spray method describedherein using a nitrogen-oxygen mixture plasma gas mixture followed by adense fluid spray treatment will selectively clean all surfaces of theexemplary optical bench, as well as prepare the interior of the assembly(222) for subsequent sealing.

[0110]FIG. 15 is an exemplary selective cleaning application comprisinga fiber optic connector substrate. A fiber optical connector (234)contains a ferrule assembly (236), which comprises a ceramic bodyhousing a cladded optical fiber. As shown in the figure, the front sideof the ferrule (236) has critical surface features exposed including aceramic face (238), optical fiber cladding face (240), and the opticalfiber face (242). Following operations such as polishing, these facesbecome heavily contaminated with polishing particles and residues. Thepresent method using a plasma and a dense fluid spray in combination maybe used to selectively clean these critical surface features withoutscratching, etching or otherwise damaging the surfaces and opticaltransmission performance of this device.

[0111]FIG. 16 is an exemplary selective cleaning application comprisinga MEMS wafer substrate. As shown in the figure, discrete dies (244)manufactured on the wafer substrate (246) using micromachining as wellas more conventional IC circuit manufacturing techniques requirefrequent cleaning in between manufacturing steps. As shown in thetopside figure (248), surface features such as trenches, vias, gears,and beams present significant surface cleaning challenges requiring amultiplicity of cleaning energies and techniques. The instant methodcomprising plasma, dense fluid, EHD, and UV/O₃ treatments herein willprovide the energies necessary to remove all varieties commonpatterning, micromachining, and plasma etching residues encountered inthe MEMS manufacturing process without damaging delicate surfacefeatures as depicted in the figure (248).

[0112] Finally, having thus described more general examples of precisionsubstrate surface applications for the present invention, the followingdiscussion in relation to FIG. 17 describes a cluster cleaning tool forperforming all possible instant dry cleaning methods described hereinusing the present invention.

[0113]FIG. 17 is a graphic showing an exemplary combinationalcluster-cleaning tool for performing sequential dry surface treatmentsusing the present invention. As shown in the figure, a cluster-cleaningtool may comprise a hexagonal workstation (250), having at its centerrobotic transfer robotics (252). Clustered about the substrate transferrobotics (252) are five selective treatment modules as follows; a vacuumor atmospheric plasma treatment module (254), a dense fluid solid,liquid or supercritical fluid treatment module (256), an EHD treatmentmodule (258), a UV/O₃ treatment module (260), and an inspection module(262), which can be a vision inspection system, OSEE system, and otherpossible surface inspection techniques. Integrated with the exemplaryworkstation (250) is an incoming substrate conveyor (264) and anoutgoing processed substrate conveyor (266). The system thus describedmay be programmed to perform any of the instant dry substrate surfacecleaning methods described herein.

[0114] Additional real-world examples of use and instant dry cleaningand modification methods developed using the present invention are asfollows.

[0115] Lapped and Polished Sapphire Wafer Cleaning and Stain RemovalMethod

[0116] Remove gross polishing agents and water residues:

[0117] 1. CO₂ steam flush—200 C., 120 psi, 2-3 minutes

[0118] Remove fine particle and thin film residues:

[0119] 2. CO₂ snow spray—80 psi, 5-30 seconds

[0120] Degrade surface stains:

[0121] 3. Vacuum plasma treatment—100 mTorr, 200 watts, Ar/O₂atmosphere, 5 minutes

[0122] Remove plasma degraded contaminant residues:

[0123] 4. Co₂ snow spray—80 psi, 5-30 seconds

[0124] Implantable Polyester Graft Substrate Cleaning, Biocompatibility,and Sterilization Method

[0125] Remove extractable contaminants:

[0126] 1. Supercritical CO₂ extraction—2500 psi/60 C, 60 minuteextraction cycle

[0127] Degrade (oxidize) surface residues and biological activity:

[0128] 2. Vacuum plasma treatment to degrade monomers and reduce surfaceparticle adherence—500 mTorr, 200 watts, N₂/O₂ atmosphere, 5 minutes

[0129] Remove plasma degraded contaminants:

[0130] 3. Liquid CO₂ rinse to wash plasma treated particles, 1200 psi,25 C., 20 minute cycle

[0131] Activate surface to improve biocompatibility:

[0132] 4. Vacuum plasma treatment to degrade monomers and reduce surfaceparticle adherence—500 mTorr, 200 watts, Ar/02 atmosphere, 5 minutes

[0133] Soiled Inspection Boroscope Cleaning and Sterilization Method

[0134] Remove gross biological fluids and residues:

[0135] 1. Water rinses and drain—30 C., 20 psi, 2-3 minutes

[0136] 2. CO₂ steam flush—250 F., 120 psi, 2-3 minutes

[0137] Degrade adhering thick films and biological soils:

[0138] 3. Vacuum plasma treatment—100 mTorr, 200 watts, Ar/O₂atmosphere, 5-60 minutes

[0139] Remove plasma degraded residues and large particles:

[0140] 4. Liquid CO₂ spray—900 psi, 25 C., 10 minutes

[0141] Remove small particles:

[0142] 5. CO₂ snow spray—80 psi, 5-30 seconds

[0143] Although the present invention has been illustrated and describedusing specific substrates, substrate surfaces, substrate treatmentapplications and exemplary treatment apparatuses, it will be fullyunderstood by those skilled in the art that many additional examples ofuse, variations of the method described herein, and apparatuses foremploying said instant dry cleaning methods for a variety of substratesand substrate surfaces are possible while still remaining within thescope of the present invention.

I claim:
 1. A method for cleaning the surface of a substrate having asurface contaminant comprising contacting the surface of a substratehaving a surface contaminant with a plasma and with a dense fluid toclean the substrate surface.
 2. The method in accordance with claim 1wherein the plasma is an atmospheric plasma.
 3. The method in accordancewith claim 2 wherein the plasma is contacted at a temperature of lessthan 60° C.
 4. The method in accordance with claim 2 wherein the densefluid is supercritical carbon dioxide, liquid carbon dioxide, solidcarbon dioxide or solid argon.
 5. The method in accordance with claim 4wherein the surface having a surface contaminant is first treated withthe plasma and then treated with the dense fluid.
 6. The method inaccordance with claim 4 wherein the surface having a surface contaminantis simultaneously treated with the plasma and with the dense fluid. 7.The method in accordance with claim 3 further comprising contacting thethus cleaned surface with a low pressure plasma.
 8. The method inaccordance with claim 7 wherein the low pressure plasma is contacted ata pressure in the range from 1 to 500 millitorr.
 9. The method inaccordance with claim 3 wherein the substrate is a wafer, die, CMOSimage sensor, fiber optic connector, optical fiber, optical bench,optic, IC test socket pad, flexible polyimide gold circuit, PCB rework,lead frame bond pad, photodiode or medical device.